1. Field of the Invention
The present invention relates to a module cover for an air bag device for a driver which is mounted on a steering wheel of an automobile, a module cover for an air bag device for a passenger which is installed in the instrument panel or the like of an automobile, and a module cover for an air bag device for a rear seat occupant. More particularly, the present invention relates to an air bag device improved so that the module cover is torn to open exactly along a tear line provided thereon when the air bag extends.
2. Description of the Related Art
A conventional air bag device for a driver and the module cover thereof will be explained with reference to FIGS. 3 to 6.
As shown in FIG. 3, an air bag device 12 for a driver is provided at the central portion of a steering wheel 10 of a motor vehicle. The air bag device 12 for a driver is composed of an inflator, an air bag attached to an adapter plate which is generally called a retainer, and a module cover 14 provided in such a manner as to cover the inflator, the air bag and the retainer. Engaging portions 18, 20, 22 and 24 which are engaged with spokes 16 are formed on the side surfaces of the module cover 14, as shown in FIGS. 4 and 5.
The module cover 14 is formed of a synthetic resin. The module cover 14 may include a reinforcing material such as a mesh embedded therein. The module cover 14 is provided with tear lines along which the module cover 14 is to be torn open.
The tear lines include a first tear line 28 provided along the center of the module cover 14 in the transverse direction of a vehicle body, i.e. in the widthwise direction of the module cover 14, and second tear lines 30, 32 provided along the side edges of the module cover 14 in the longitudinal direction of the vehicle body, i.e. in the lengthwise direction of the module cover 14. When the "longitudinal" direction or the "transverse" direction of the vehicle body is referred to in this specification, it is assumed that the steering wheel 10 is at the position for a straight forward drive. The second tear lines 30, 32 are provided along and at a short distance away from adapter members 34, 36 for fixing the module cover 14 to the retainer (not shown). The thickness of the module cover 14 at the portions 100, 200 outside the second tear lines 30, 32 is equal to the thickness of the portions 300,400 between the second tear lines 30, 32. The thickness of the adapter members 34, 36 is also equal to the thickness of these portions 100, 200, 300 and 400.
The tear lines are produced, for example, by forming a continuous groove or aligned hollows along lines where the module cover is to be torn open. In some module covers, the depth of the groove is increased at some portions. In some two-layered module covers consisting of a hard-material layer and a soft-material layer, slits are formed on the hard-material layer along the lines along which the module cover is to be torn open.
At the time of a collision of an automobile with such an air bag device installed therein, the inflator (gas generator) is operated so as to extend the air bag. The module cover 14 is first torn open along the first tear line 28, and the tearing operation propagates to the second tear lines 30, 32. Accordingly, the module cover 14 is largely torn open as indicated by the arrows of two dotted line in FIG. 3, so that the air bag extends largely in a cabin.
FIG. 9 is a perspective view of a conventional air bag device for a passenger or a rear seat occupant seen from the back side thereof, and FIG. 10 is a schematic sectional view of the air bag device shown in FIG. 9 in the operational state.
A module cover 70 is substantially rectangular, and adapter members 71, 72 are attached to the back surface of a plate-like cover member (lid portion) 70a in such a manner as to extend in the longitudinal direction of the module cover 70.
The module cover 70 is attached to the front surface of a container 73 of the air bag device, as shown in FIG. 10. When an inflator 74 is operated so as to inject gas for extending an air bag 79, the module cover 70 is torn open along the first and second tear lines 75, 76, and the air bag 79 extends largely in the cabin.
The first tear line 75 extends in the transverse direction of a vehicle body and the second tear lines 76 extend in the direction (longitudinal or vertical direction of the vehicle body) orthogonal to the transverse direction. In the module cover 70 shown in FIGS. 9 and 10, the tear lines 75, 76 are provided in the shape of H. In contrast, in a module cover 70A shown in FIG. 11, tear lines 77, 78 are formed in the shape of U. In this case, the module cover 70A is torn open as shown in FIG. 12 when the air bag extends.
The thickness of each cover members (lid portions) 70a, 70b of the module covers 70 and 70A is uniform as a whole. Each of the module covers 70 and 70A is formed of a synthetic resin. A reinforcing material such as a mesh may be embedded therein.
As described above, in a conventional air bag device, when the air bag extends, the module cover is first torn open along the first tear line, and the tearing operation propagates to the second tear lines, so that the module cover is largely torn open.
However, since the first tear line and the second tear lines are approximately orthogonal at the intersecting portions, there is a fear that the tearing operation progressing along the first tear line progresses inertially on the module cover in the transverse direction of the vehicle body without changing the course.